Botlane Sion is viable because of two things :
-His survivability, counting his W passive, which grants some HPs per creeps but most importantly 15 HPs per takedowns. This passive makes Sion still tanky enough to do his damages while only building offensive items.
-His damages. Simply, you want to fight as much as possible with Sion (you can easily force fights with Nautilus support) Sion's Q does a lot and can litteraly one shot every ADC or squeeshie support. Finally, Sion's passive makes him a threat even when burst down, crit makes a lot of sense when the passive AA comes because of the max-health damages doubled by the crit.

Prowler is good use when alive, but its even better when used dead, putting Sion behind the target helping him finish the kills or even makeing new ones out of nowhere. There was a time when prowler could be used on creeps, making his E + prowler one of the most surprising combo but its not doable today, sadly, Prowler can only be used on champions nowadays.

Note that teleportation is an essential tool to kill, here's an example :
You start a 2v2 against the enemy botlane with your Nautilus support, you end up diying and not finishing one of the targets and your Nautilus dies too. Don't be sad, the enemy ADC that survived will continue farming the wave, giving you the chance to TP (don't forget that during Sion's passive, the death timer is already running) really fast after the fight, making a suprise attack, giving you a second chance (or even third chance since passive is already a second chance) to kill your oppenent.

Thank you for reading this, I hope you like the way I see Sion being played, I'm open for advices or comments so don't be shy to send me a message about this build !